Concept of Surface Roughness, Surface Finish and Surface Integrity in Machining

Appearance as well as several tribological properties (related to friction, wear and lubrication) of solid surface depends greatly on the level of finish of the concerned surface. Various machining processes including conventional processes (like turning, milling, drilling, etc.), finishing processes (like grinding, honing, lapping, etc.) and non-traditional manufacturing processes (like USM, EDM, LBM, etc.) are predominantly used to provide proper dimension and smooth surface by removing excess layer of material. Surfaces generated by any of such manufacturing processes inherently contain several errors. Overall quality of a solid surface can be judged by three common indices namely surface roughness, surface finish and surface integrity.

Concept of surface roughness

Surface roughness is a widely used index to assess quality of the product processed via any manufacturing process including casting, forming, machining, rapid prototyping, etc. It basically refers to the localized deviation of a surface from its nominal level because of presence of irregularities and asperities.

Surface roughness is one quantifiable character that basically displays height of the asperities present on the surface over a length or an area. In conventional tool based machining processes, scallop marks or feed marks are the prime sources of rough surface and thus height of such unmachined portion can provide the value of surface roughness.

Roughness value can be estimated with the help of few cutting parameters and tool geometry information. However, such mathematical analyses are based on many assumptions which rarely comply with actual machining. In some cases many neglected factors become predominant and thus actual roughness changes greatly from the estimated one. Thus it is also necessary to directly measure roughness of the machined surface and thus can be done with the help of various instruments like profilometers, microscopes, etc. You may further read following articles.

Concept of surface finish

Surface finish indicates quality of machined surface with the help of various attributes like fine, rough, good, poor, etc. It cannot be determined quantitatively; instead it can be expressed qualitatively. Value of surface roughness can indirectly indicate level of surface finish. In fact, they are reciprocal. For example, average roughness of a turned surface is 20µm (value and unit to quantify) or this surface rough (attribute to qualify).

Improving surface finish is one common aim for most of the manufacturing processes. However, the process dependent behavior of surface roughness makes it very difficult to minimize it to the lowest extent as such steps can considerably degrade material removal rate. Influences of various uncontrollable parameters also enshroud a straightforward solution for improving finish quality. For further information, you may read following articles.

Concept of surface integrity

Surface integrity covers the overall surface condition of the workpiece processed by any manufacturing processes. It consists of a wider spectrum of surface characteristics which also includes surface roughness. Apart from basic topographic information about the surface, several metallurgical, mechanical and physical properties, which may affect during manufacturing processes, also come under the preview of surface integrity.

In general, surface integrity consists of topological attributes like surface roughness, waviness, irregularities, form error, etc. and also metallurgical characteristics like residual stress, sub-surface defects including cracks, microstructure, crystallographic orientation, phase change, recrystallization, few mechanical, chemical and physical properties, etc. Basically surface roughness is a subset of surface integrity.

References

  • Book: Advances in Manufacturing Technology XV by D. T. Pham, S. S. Dimov and T. O’Hagan (Professional Engineering Publishing Limited).
  • Proceedings: K. S. Umashankar, B. J. Manujesh and N. A. Jnanesh; Recurrence Qualification Analysis – A Tool to Analyze the Surface Finish During Machining; Advances in Mechanical Engineering; 2010.
  • Book: Machining and Machine Tools by A. B. Chattopadhyay (Wiley).
  • Book: Metal Cutting: Theory And Practice by A. Bhattacharya (New Central Book Agency).